Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15July2015) This issue is about box boat stability. The 1August issue will continue the topic.
THE BOOK IS OUT!
BOATBUILDING FOR BEGINNERS (AND BEYOND)is out now, written by me and edited by Garth Battista of Breakaway Books. You might find it at your bookstore. If not check it out at the....
ON LINE CATALOG OF MY PLANS...
...which can now be found at Duckworks Magazine. You order with a shopping cart set up and pay with credit cards or by Paypal. Then Duckworks sends me an email about the order and then I send the plans right from me to you.
Chuck Leinweber's new Toon19, Looneytoon, on the recent Texas 200. Is that a dazzle camo paint job or an attempt to use up those old partial cans of paint?
118 E Randall,
Lebanon, IL 62254
Send $1 for info on 20 boats.
Box Boat Stability
BACK TO REND LAKE...
...I got a bit of a surprise at Rend Lake this year when we took the Jukebox3 out for a power test...
There were four us going, a total of 900 pounds of people we figured. The empty weight, with ballast, of the boat is about 1600 pounds and we pretty much had it maxed out weight wise.
WHAT HAPPENED CAN HAPPEN TO ABOUT ANY BOAT...
...SO listen up. Even fairly large ships have found themselves in this situation.
We pushed off deep enough to drop and start the motor and back out. I told the gallant crew that it would steer best with the leeboard down a bit. The captain was at the remote steering station in the starboard side of the house....
...then three full sized men reached over the starboard gunnel to handle the leeboard, the boat lurched to that side and we ALMOST capsized! Or so it seemed to me before we scrambled back in time.
I was quite surprised at this. A few of this type of boat have been around a while and no one ever said anything about this action. Probably because they never had four men reaching over the same side of the boat at the same time. Anyway, with things settled down we continued our journey...
DIGGING BACK IN TIME...
I found the old hullform stability model I used when I figured this design a long time ago (it looks funny because of the constaints of the program but the immersed part is correct)...
The model on file was for a normal sailing situation, at 2000 pounds total with two crewmembers seated properly and behaving themselves. It predicted the following righting curve...
So it predicts the boat will reach a maximum righting power at 28 degrees of heel. The Gz, or righting arm, at that time is .66ft. So the righting moment that is trying to push the boat back upright then is .66x2500=1650 ft pounds. Hullforms does not do solutions with weights offset from the centerline but we can do that by hand. If the boat were symetric (not exactly true but close) and the deck 5ft wide and the crew of 900 pounds total all reached over the same gunnel at once then the moment they apply is about 900x2.5=2250 ft pounds. And that is well beyond the righting ability of the hull. It would indeed go past 28 degrees and over on its side until it relieved itself of the off center crew. So maybe I was right to panic.
Now keep in mind that the skipper here is already to one side of the boat as the motor controls are attached to the hull side. And I suppose the next question might be just how many men can reach over the side without exceding the allowable 1650 ft pounds. So it would be 1650/2.5=660 pounds, about three men.
I THINK THIS SORT OF THING APPLIES TO ANY BOAT...
...but some, with wide flat decks, beg for it more than others. I know large sight seeing boats have capsized as the customers all ran to one side to see the beautiful sunset. And, alas, more recently a large boat full of refugees capsized as a rescue boat pulled alongside which caused the unforturnates to all rush to that side of the boat at once. This is a flat bottomed boat with a wide raised deck which sort of encourages walking around and might be the worst case.
ACTUALLY, IT CAN GET WORSE
Recall that this hullform cg data was based on a crew sitting responsibly to keep the cg low. What if they all stand up? Let's say two of the men, 500 pounds total, stand up on the aft deck. The cg location of the total is raised from 2' above the bottom to 2.5'. Hullform predicts a new righting moment curve like this...
It is quite a bit worse. The max righting moment now is .43x2500=1075 footpounds and it occurs at 22 degrees of roll. Now 2 men on the rail will put it over. Let's say two men stepping on board from a dock at the same time. So you see you must be careful.
Actually, as we motored around in the Jukebox we thought we had it figured out and, after all, we were experienced with this now. But as we gained the dock to put the boat back on its trailer, we all reached over the side at once for the dock lines...well we panic properly again and learned the lesson a second time.
I am going to delve into this sort of boat again, with more detail, in a future essay....
MIKESBOAT, SAILBOAT, 17' X 5.5', 450 POUNDS EMPTY
Mikesboat is a big Piccup Pram. It is slightly narrower in proportion. The idea was to have something like Piccup that was large enough to take the family so she has two bench seats 8' long. It should be a good expedition solo boat with a boom tent fitted over the cockpit. Mikesboat has the Piccup layout with buoyancy/storage chambers fore and aft but now there is also enough room for a small motor well across the stern. As a rule something like this needs maybe 2hp to run at hull speed and even then there would be some extra. This shape of hull with multichines has proven good in rough water with fair speed.
Sail rig is the same 96 square foot sharpie sprit rig that I used on AF3 and a few other designs (actually it is the mainsail from a Bolger Jinni design I built 20 years ago). This is a change from the balanced lug rig that Piccup uses but Mike had good experiences with his Mayfly12 which used the sharpie rig and that is what he wanted. I would expect the sharpie sprit to be slightly better to windward than the lug but not by a lot and the sailmaker's talents might be the deciding factor here. There is no doubt that the sharpie sprit mast is perhaps 50% longer than a similar lug sail might need.
Mikesboat uses taped seam construction. Takes nine sheets of 3/8" plywood and three sheets of 1/2", compared to Piccup's fives sheets o 1/4". So you see that scaling up a boat makes for a lot more wood and weight.
I dunno if Mike ever built his boat but Bill Moffitt in Atlanta made one to run the Texas200. He is an experienced builder and sailor and made a thorough job of it in a big rush towards the end. Launching at the Texas200 was its first time in the water. His wife sewed the sails the day before he left for the cruise, and this was her first set of sails! The sail rig he wanted was a lug main with a sharpie mizzen. I drew those up for him and they are now included in the plan set. Here is his boat under sail:
Here is another photo of the boat beached beside Chuck Leinweber's mothership Caprice (which was designed originally for Bill Moffitt!) at the end of the cruise:
The cruise involved camping through several nights and Bill did something I have always thought about but had never seen. He pitched a standard tent in the large Mikesboat cockpit. A lot cheaper than a custom boat tent, for sure, but it takes a big cockpit to handle it. Bill also had some sort of center platform to fill the space between the long bench seats:
Well, three cheers for Bill and son Paul!!! As far as I know the 200 mile cruise went off without a hitch, the boat right on. But I suggest that most of us couldn't get away with going on a long cruise with an untested design.
And here is another Mikesboat, owned by David Chase...
Plans for Mikesboat, showing both sailrigs, are $45.
Some of you may know that in addition to the one buck catalog which now contains 20 "done" boats, I offer another catalog of 20 unbuilt prototypes. The buck catalog has on its last page a list and brief description of the boats currently in the Catalog of Prototypes. That catalog also contains some articles that I wrote for Messing About In Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3. The both together amount to 50 pages for $4, an offer you may have seen in Woodenboat ads. Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currency that I can admire but not spend.) I'm way too small for credit cards.
I think David Hahn's Out West Picara is the winner of the Picara race. Shown here on its first sail except there was no wind. Hopefully more later. (Not sure if a polytarp sail is suitable for a boat this heavy.
Here is a Musicbox2 out West.
This is Ted Arkey's Jukebox2 down in Sydney. Shown with the "ketchooner" rig, featuring his own polytarp sails, that is shown on the plans. Should have a sailing report soon.
And the Vole in New York is Garth Battista's of www.breakawaybooks.com, printer of my book and Max's old outboard book and many other fine sports books. Beautiful job! Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat. But he has used it extensively on his Bahamas trip towed behind his Cormorant. Sort of like having a compact car towed behind an RV.
And a Deansbox seen in Texas:
Another prototype Twister is well along:
And the first D'arcy Bryn is to the point the builder can sit and relax in it and imagine boating. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....
The first Jukebox3 is on the (cold) water. The mast is a bit too short - always make your mast too long. A bit more testing will be nice...
A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) Double layer bottom on and glassed, hull returned to upright...
AN INDEX OF PAST ISSUES
THE WAY BACK ISSUES RETURN!
MANY THANKS TO CANADIAN READER GAETAN JETTE WHO NOT ONLY SAVED THEM FROM THE 1997 BEGINNING BUT ALSO PUT TOGETHER AN EXCELLENT INDEX PAGE TO SORT THEM OUT....
THE WAY BACK ISSUES
1aug14, Taped Seams, Cormorant
15aug14, Plywood Butt Joints, Paulsboat
1sep14, Rowing 1, Vireo
15sep14, Rowing 2, Philsboat
1oct14, Guessing Weight, Larsboat
15oct14, SailOK2014, Jonsboat
1nov14, Chine Runners, Piccup Pram
15nov14, Lugsail Rigging, Caprice
1dec14, Sail Area Math, Ladybug
15dec14, Poly Laminates, Sportdory
1jan15, Sharpie Spritsail, OliveOyl
15jan15, Knockdown Recovery, Dockbox
1feb15, Mike Monies, Laguna
15feb15, Cartopping, IMB
1mar15, WeeVee Lessons, Vole
15mar15, Bulkhead Bevels, Frolic2
1apr15, Capsize Lessons, Riverrunner
15apr15, Hollow Spars, Slam Dink
1may15, Boat Costs, Blobster
15may15, Small Boat Rudders, Roar2
1jun15, Emergency Flotation, RB42
15jun15, Thailand Mixer Cruise, Mixer
1jul15, Rend Lake 2015, Musicbox3
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Puddle Duck Website
Brian builds Roar2
Herb builds AF3
Herb builds RB42
Barry Builds Toto
Table of Contents